A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols such as methanol can be used in specific applications. Fuel cells are different from batteries in that they require a constant source of fuel and oxidant to operate, but they can produce electricity continually for as long as these inputs are supplied.
There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows ions to move between the two sides of the fuel cell. Catalysts are required to aid the release of ions and electrons. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. The resulting electricity may thus be used to power a suitable electronic device.
As the voltage produced by a fuel cell is relatively low, it is common to arrange existing conventional fuel cells in series to produce a higher voltage output. To achieve this in a convenient manner, the cells are typically arranged in a stack. Hydrogen and oxygen are provided to each cell in the stack and the water by product removed from each cell.
The present application is directed generally at a type of fuel cell referred to generally as Proton Exchange Membrane (PEM) fuel cell. The present application can also be directed generally to electrolysers. A PEM fuel cell comprises a Membrane Electrode Assembly (MEA) 1, comprising of a proton exchange membrane 2 sandwiched between two electrodes (the anode 4 and the cathode 6), which contain the electro catalyst 8, 10 as shown generally in FIG. 1. Attached to each electrode is a Gas Diffusion Layer (GDL) and at each end a flow plate is provided (not shown). The flow plate which is traditionally solid material coated in a corrosive resistant, conducting material has many functions within the PEM fuel cell but its primary aim is to act as a manifold to supply fuel and oxidant gases to the MEA reactive sites and this makes the flow plate a vital component for the PEM fuel cell.
The flow plate acts as a gas distributor and allows for the supply and control of fuel and oxidant. At the same time it allows for an exit for waste water through the open channels and waste heat through the conductive flow plate material. Machined Channels are provided in the flow plate in order to reduce gas transport losses. The flow plate may also house cooling tubes to manage the temperature of the fuel cell. However the necessity of an intricate design has an effect on the cost of the fuel cell; compatible materials and machining costs escalate prices.
The overall efficiency of the fuel cell depends extensively on the performance of the flow plate used. Using solid metal or carbon or graphite for the flow plate is both expensive and at the same time adds considerable weight. As a result, several types of materials have been employed for flow plate construction including, for example, polymer-coated metal sheet, electro & flexible graphite, carbon-carbon composite and thin metallic sheets.
A significant design consideration is the interface between the Gas Diffusion Layer (GDL) and the flow plate. With conventional flow plates 21, 22, such as for example arrangements those in which the flow fields are machined serpentine or parallel structures in a flow plate, the dominant reactant flow 24, 26 (flow shown generally with arrows) is in a direction parallel to the electrode surface. In this configuration 20, illustrated in FIG. 2, the reactant flow to the catalyst layer is predominantly by molecular diffusion through the GDL 4, 6. This can lead to large concentration gradients across the GDL and mass transfer limitations because of the small channel dimensions, laminar gas flow and the inherent slow molecular diffusion process.
Interdigitated flow plates 30, as shown in FIG. 3, provide convection velocity normal to the electrode surface for better mass transfer and enhanced water removal from the channels and GDL. This design employs flow plates 31, 32 with dead ended flow channels, which are not continuous from inlet manifold to exit, so that the reactant flow is forced under pressure to go through the GDL. This provides enhanced performance at high current density operation. However large pressure losses and high parasitic power (due to increased gas flow pressure) are characteristics of this type of flow field, which may limit this application to smaller stack sizes.
Open Pore Cellular Foam (OPCF) is a relatively new class of cellular material with the ability to be manufactured with tailored mechanical, thermal, acoustic and electrical properties by varying the material's relative density and cell morphology. OPCF material can provide great benefits to solve many engineering problems and at present it has many applications in filter systems, heat exchangers and more recently in the electrodes of some electrochemical devices; super capacitors, batteries and electrolysers.
As the name suggests these materials have an open pore structure composed of isotropic pores which are connected to each other by ligaments. The array of pores forms a solid homogonous matrix, having the same properties of the parent material but at the fraction of the weight. These materials are mainly manufactured by casting or foaming, leaving the final open pore foam material.
Porous Metal Foam (MF) & Reticulated Vitreous Carbon Foam (RVCF) materials are two common classes of OPCF. A variety of metals including; aluminium, copper, tin, zinc, nickel, silicon, stainless steels with high nickel or chrome content, silver and gold, can be made to produce an open pore MF. Advanced combinations can be achieved through secondary coating processes. RVCF is an OPCF material composed solely of vitreous carbon. As its name implies, vitreous carbon is a form of glass-like carbon that combines some glass properties with those of normal industrial carbons. The porosity of OPCF can be tailored and this gives the benefit that the use of metal foam may negate the need for an additional gas diffusion layer and also be used to support the catalyst in a region adjacent to the membrane and thus serve the purpose of an electrode.
Kumar and Reddy “Application of Metal Foam in the flow field Distributor of Polymer Electrolyte Membrane Fuel Cell Stack” Abs. 1060, 204th Meeting, 2003 The Electrochemical Society proposes a flow plate 40 in which metal foam 42 is placed in the flow channels, acting as electrodes, in place of the machined channels within the existing bipolar flow plate. In this arrangement, which is shown in FIG. 4, the central area 46 of the bipolar flow plate 44 is machined out or otherwise removed to form a recess in which metal foam is then placed. The representation of FIG. 4 is for a bipolar structure and hence the structure is replicated on opposing sides of the bipolar flow plate 44. Kumar and Reddy design consists of a bipolar flow plate 44, and this provides a separator or gas barrier between adjacent cells as shown in FIG. 4. Catalyst may be provided with the metal foam in regions adjacent to the membrane 2 such that the metal foam acts as an electrode. Thus in the construction shown there is metal foam 42, 43 provided on opposing sides of the membrane 2, in place of the machined channels within the existing bipolar flow plate. Whilst the use of foam in place of the machined channels offers advantages, the construction of Kumar and Reddy remains relatively bulky and heavy.